The recent advances in the field of optical communication with new, more complex, data modulation formats as a key technology has created a need for optical waveform characterization tools that are capable of extracting more information from the waveform than simply its power as a function of time. Encoding data onto the optical carrier by modulation of the optical-field phase and the optical-field amplitude becomes increasingly attractive and seems to be a technology that will contribute to increase the capacity of future fiber optic communication links. However, measurement of the complete electrical field of the optical signal, which is required to visualize both its phase and amplitude information, requires coherent detection techniques that utilize a reference phase at the measurement point, for example a continuous wave (CW) laser emitting local oscillator light source (LO). The mixing of the input optical signal with a reference optical signal (LO) will open up the possibility of measuring the time-varying phase changes of the input optical signal relative to the LO signal.
Coherent detection is not a novel technology. In fact, it was extensively studied during the 1980's and the technology was proposed as a solution for high-sensitivity signal detection. However, implementation was difficult and with the advent of Erbium-doped fiber amplifiers (EDFA), the commercial deployment of coherent systems has been delayed. Nevertheless, research has continued in the field and the basic understanding of coherent detection systems has been summarized in references such as “Fiber-optic communication systems” by G. P. Agrawal (Wiley, 3rd ed., 2002). Recently, coherent detection approaches have attracted renewed interest, driven by the need for more spectrally-efficient modulation formats, as well as the availability of high-speed electronic processing for post-compensation of transmission-created impairments.
The transition towards novel advanced modulation formats for optical communication that incorporate modulation of both amplitude and phase has created a need for new measurement technologies that are capable of measuring the time-dependent electrical field of the optical signal under test. For high-speed optical data signals, the measurement system also requires a high measurement bandwidth for accurate reconstruction of the optical signal under test. Digital sampling is a technology that can provide enough measurement bandwidth for the high-speed optical signals, and can be used in conjunction with coherent detection to provide high-speed electrical field measurements of the optical signal under test.
Digital sampling is a technique used to visualize a time-varying waveform by capturing quasi-instantaneous snapshots of the waveform via, for example, a sampling gate. The gate is “opened” and “closed” by narrow pulses (strobes) in a pulse train that exhibit a well-defined repetitive behavior such that ultimately all parts of the waveform are sampled. The sampling implementation can either be real-time or equivalent-time, where real-time sampling refers to the case where the sampling rate is higher than twice the highest frequency content of the waveform-under-test (Nyquist sampling), while equivalent-time sampling uses an arbitrarily low sampling rate. However, equivalent-time sampling requires the measured waveform to be repetitive (in order to provide accurate signal reconstruction)—a fundamental limitation when compared to real-time sampling.
The prior art includes several implementations that facilitate coherent measurement of the electrical field of an optical input signal carrying optically-encoded data by utilizing digital sampling, coherent mixing with a reference signal and subsequent signal processing for signal reconstruction and visualization. Representative prior art solutions will be outlined here, with particular limitations identified that are addressed by the present invention.
FIG. 1 shows a prior art arrangement for measurement of the electrical field of an optical input signal S. As shown, optical input signal S and a reference local oscillator optical signal LO from a laser source 14 are applied as separate inputs to a 90° optical hybrid 16. Optical hybrid 16 mixes optical input signal S with the four quadrature states associated with reference signal LO in the complex-field space. The operation of optical hybrid 16 therefore generates a set of four mixed fields, representing the complex field sums S+LO, S−LO, S+jLO and S−jLO, as shown.
Thereafter, the, pair of field sums S+LO and S−LO as applied as inputs to a first balanced detector 18, which will generate an electrical signal output representative of the difference between the two signals. Similarly, the pair of field sums S+jLO and S−jLO are applied as inputs to a second balanced detector 20. By square law detection of the four fields in balanced detectors 18 and 20, the two output detector signals (electrical currents) are expressed as:I1(t)=4|S(t)∥LO|cos(ωIFt+φS(t)+φLO,1), andI2(t)=4|S(t)∥LO|cos(ωIFt+φS(t)+φLO,2),where the intermediate frequency IF related term ωIF is defined as ωS−ωLO, which is the angular frequency difference between the signal field and the LO field. The term φS(t) represents the time-varying phase of optical input signal S, and the quantity (φLO,1−φLO,2) represents the relative phase shift of the optical reference signal LO between the hybrid outputs. Advantageously, this induced relative phase shift will be selected to be π/2 for an optical hybrid such as optical hybrid 16 (thus termed as a “90° optical hybrid”), although in general other phase shifts may be employed, provided that they are not integral multiples of π.
The output currents from balanced detectors 18 and 20 are then amplified by amplifiers 22 and 24 before being digitally sampled in analog-to-digital (A/D) converters 26 and 28. Finally, the acquired batches of samples from I1(t) and I2(t) are applied as inputs to a signal processor 30 in order to recover a visualization of the electrical field of optical input signal S. Inasmuch as the arrangement of FIG. 1 uses two separate laser sources for optical input signal S and reference signal LO, the intermediate frequency will be non-zero (i.e., ωIF≠0). As a result, the value of the IF needs to be calculated in order to extract φS(t), which represents the phase modulation of interest of the signal. There are several algorithms available for extracting ωIF in the prior art, see, for example, US Published Application 2006/0245766, authored by M. G. Taylor and published on Nov. 2, 2006. With the IF recovered, it is straightforward to then extract both amplitude and phase information for optical input signal S and visualize the measured signal as, for example, a constellation diagram.
This prior art coherent detection technique as shown in FIG. 1 requires the use of electronic sampling technology (A/D converters 26, 28) and thus has at least one significant drawback associated with the bandwidth limitation of the electronic A/D converter and digital sampling circuits. The highest available analog bandwidth in high-speed A/D converters is today typically <20 GHz and hence the maximum measurable signal “baud” (i.e. symbol rate) is generally less than 30 GBaud.
In contrast to electrical sampling, optical sampling is a proven technology that can provide extremely high bandwidth. Traditionally, optical sampling has been used to measure the time-varying optical power of an optical input signal with very high temporal resolution, but very few optical-sampling implementations are capable of measuring the complete electrical field of an optical signal.
FIG. 2 shows an exemplary prior-art arrangement for a coherent detection linear optical sampling system that is capable of measuring the complete electrical field of the optical input signal. In this case, optical input signal S is mixed in an optical hybrid 36 with coherent light serving as a reference local oscillator (LO) signal originating from a pulsed sampling laser source 34. The main difference between the prior art linear sampling system in FIG. 2 and the electronic sampling system in FIG. 1 is the utilization of “pulsed” reference LO signal in the arrangement of FIG. 2. In contrast, a continuous wave (CW) source 14 is used in the prior-art arrangement of FIG. 1. The use of a pulsed sampling laser provides a fast gating functionality that is independent of the limited bandwidth of an A/D converter. By reducing the optical-sampling rate to well below the analog bandwidth of the A/D converter, the measurement bandwidth of the system will be dictated only by the temporal resolution of the optical-sampling gate (roughly the pulse width of the pulsed source 34).
As before, the four output mixed electric-field signals from optical hybrid 36 are applied as inputs to a pair of balanced detectors 40 and 42. The detector currents are then amplified by amplifiers 44 and 46 and applied as separate inputs to an A/D converter 38. In this prior-art linear sampling system, A/D converter 38 needs to operate at the same sampling rate as the pulse rate of the sampling laser 34. To accomplish this, a photodetector 48 and a pulser circuit 50 are coupled between sampling laser source 34 and A/D converter 38 and are used to create a clock signal that synchronizes the sampling rate of sampling laser 34 with the sampling rate of A/D converter 38. With acquired batches of samples of the photodetector currents from A/D converter 38, the required signal processing needed in order to reconstruct the original waveform is similar to that described for the electrical sampling case in FIG. 1 and is not explicitly illustrated in FIG. 2.
There remain, however, a few drawbacks with this hardware implementation, particularly related to strict wavelength requirements on the sampling pulse laser. That is, the linear optical sampling technology requires the sampling pulse spectra to overlap the optical signal spectra in order to provide distortion-free gating and coherent mixing using the same laser source. These requirements complicate the possibility of providing an optically broadband measurement system, since if the wavelength of the optical input signal is changed, the pulsed sampling laser must also adapt its wavelength (as well as adapt the required spectral shaping filter 52). Another parameter which can be challenging is the fact that the pulse-to-pulse phase change of sampling laser 34 must be small, hence a high coherence pulsed laser is required.
Thus, a need remains in the art for an arrangement capable of characterizing (visualizing) the complete electrical field of high-symbol-rate optical signals without being hampered by limited electrical measurement bandwidth or by the need for unnecessarily complicated optical-sampling pulse sources.